For example, generalized Hankel Fourier transformation is mainly used to extract a circle (touchdown zone mark of the heliport) in the image taken by a camera. This kind of method is disclosed in references such as Japanese Patent Laid-Open No. 7-91914 and Japanese Patent Laid-Open No. 1-233578.
Furthermore, the extraction of a particular configuration such as a circle can be conducted with a publicly used filter. However, it is difficult to extract a particular configuration with only an independent filter, so plural filters combined in sequence are used for extracting the particular configuration.
However, it was difficult to extract a particular configuration from a complicated figure by these sequentially combined filters.